We will examine the effect of heart rate and changes in extracellular calcium and sodium levels on the intracellular concentrations of Nai, Ki, Cai, and Mgi in isolation Langendorff-perfused ferret hearts, and later in isolated fiber bundles. Measurements of Cai, Nai, Ki, and Mgi will be made by atomic absorption spectrophotometry in a matrix of states of lowered levels of Cao and Nao using 14C-sucrose as a marker for extracellular fluid. Then, chosing a set of conditions considered as "normal" and others with marked changes in Nai, Ki, and Cai, the profiles of relative atomic abundances in local intracellular regions will be obtained by the use of energy dispersive X-ray microanalysis and also electron energy loss spectroscopy. These electron microprobe data will provide the basis for estimating intracellular concentrations in subcellular regions and promote the understanding of the role of these ions in cardiac excitation and contraction.